National Transportation Safety Board Chairwoman Deborah Hersman said the conductor and engineer saw a red "stop" signal as they approached the bridge over the Mantua Creek around 7 a.m. Friday.

"They saw the bridge was closed to waterway traffic, which they found unusual, because the bridge is normally in the open position when they approach," Hersman said Saturday afternoon.

The bridge is a moveable swing bridge, which means it rotates 90 degrees on a central pivot to allow waterway traffic to pass.

Hersman said the engineer keyed in a signal to the bridge several times, trying to obtain a green "go" signal.

The conductor left the train to inspect the bridge and told the engineer "everything looked good."

After calling the dispatcher, the engineer received approval to bypass the red signal and cross the bridge. Hersman said the two locomotives and the first five cars of the train made it across the bridge when the crew said they saw the bridge collapse.

The train was crossing at 8 mph -- the bridge speed limit is 10 mph -- when the accident occurred, Hersman said.

The crew applied the emergency brake, but by that time seven cars had derailed, with four falling into the water. Mantua Creek flows into the Delaware River about 17 miles south of Philadelphia

One of the cars, carrying about 24,000 gallons of vinyl chloride -- a chemical used in the production of plastics that often is transported as a liquid -- broke open after the impact. Approximately 180,000 pounds of the chemical leaked from the tanker forming a toxic cloud that engulfed the area.

The leak did not exceed regulatory safety limits for the long-term carcinogen, officials said, but it ultimately sent more than 60 people to a hospital, most with minor respiratory complaints.

On Saturday, 106 people were still staying in hotels after being evacuated from their homes Friday night. Residents were told they may have to stay in a hotel for up to three days.

Assemblyman John Burzichelli, a Democratic state legislator from Gloucester, N.J., said the decision to evacuate those living closest to the accident site was made after careful consideration among officials.

"We decided to be overly cautious, and that's not a bad thing to do in these circumstances."

The railroad bridge, built in 1873, also collapsed in 2009, and soon thereafter underwent what Burzichelli called "a major rebuild."

On Aug. 23, 2009, a train pulling coal cars went off the tracks after the bridge collapsed and sent 16 cars into the water.

The rebuild, according to published reports at the time, included replacing about a half-mile section of track.

A diver with Conrail, which owns and operates the rail line, speculated in the days after the incident that there had been damage to the pilings supporting the bridge.

"We're aware of the 2009 collapse and we are talking with Conrail to see what was done and how it was done," said Hersman, who also noted that the NTSB was reviewing Conrail's inspection reports.

There was no word Saturday as to when the derailed cars will be removed from the water and the bridge.

Hersman said two other cars also carried vinyl chloride and another was hauling ethanol. All of the chemicals will need to be removed from the tankers before they can be hauled away.

"We're trying to figure out how to safely remove the product from those cars, and we want to make sure the situation is safe for the community and for our workers," Hersman said.

A preliminary inspection of the two locomotives revealed no defects or exceptions, Hersman said.

New Jersey Senate President Stephen M. Sweeney, a Democrat from Gloucester, said his main concern is that the rail line infrastructure be made as safe as possible.

"This rail line carries a lot of dangerous chemicals," he said. "The rail infrastructure in the Northeast is in serious trouble. The most important thing is to ensure our rail system is modernized and safe."

Hersman said a preliminary report on the cause of the accident might be available in two or three weeks.

"With hazmat releases it might take longer because of the efforts to remediate the product. We generally produce a preliminary report in about a week, but this could be much longer depending on how the cleanup goes," she said.

An update on the situation is expected to be released Sunday, Hersman said.